Surface modification method for conductive metal material

ABSTRACT

Spherical particles having curvatures larger than those of irregularities, crystal grain boundaries, lattice defects or the like (referred to collectively as irregularities, hereinafter) on the surface of a conductive metal material are ejected at high speeds to make the particles collide against the surface of the conductive metal material, thereby repeatedly causing rapid melting and cooling at the minute points of impact of the particles, thereby changing the surface into amorphous. Then, spherical particles having curvatures smaller than those of irregularities on the surface subjected to the treatment described above collide against the surface, thereby changing the surface into amorphous and planarizing the surface.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of modifying a surface of aconductive metal material to enhance the corrosion resistance andelectrical contactivity thereof that is used to establish goodelectrical connection, a conductive metal material having a highcorrosion resistance and a high electrical contactivity, a contactmember made of the conductive metal material, and a connector that is anelectrical connecting device having the contact member.

2. Description of the Related Art

In many electrical connecting devices, such as a connector, a conductivemetal material, such as a copper alloy, that is a good conductor and hasa high elasticity and a high workability, is used for a contact memberfor establishing electrical connection. If the surface of the copperalloy is corroded, the contact resistance increases. Thus, the surfaceis plated with gold or tin to enhance the corrosion resistance.

In Japanese Patent Application Laid-Open No. 8-150483 (referred to asliterature 1, hereinafter) which is laid open in 1996, there isdisclosed a method of improving the surface hardness, abrasionresistance and electrical characteristics of an electrode tip (nozzle)used for resistance welding, such as spot welding and seam welding.According to this prior art, particles having a hardness of 1000 Hv anda particle diameter of 75 to 300 μm are ejected at a speed of 180 m/secor higher for 5 to 15 seconds and to collide against the surface of theelectrode made of a non-ferrous metal, thereby repeatedly heating theregion close to the surface of the electrode to the crystallizationtemperature or higher and letting the region cool to room temperature.In this way, recovery and recrystallization are caused at the surface ofthe electrode, thereby refining the metallographic structure. Inaddition, this literature 1 discloses that lattice defects are reducedby the recovery and recrystallization process, and the mechanical andelectrical characteristics of the surface of the electrode change.

In Japanese Patent Application laid-Open No. 62-278224 (referred to asliterature 2, hereinafter) which is laid open in 1987, there is proposeda method of increasing the abrasion resistance and fatigue strength of ametal surface by ejecting particles having a hardness approximatelyequal to or more than the finished hardness of the metal surface and adiameter of 40 to 200 μm at a speed of 100 m/sec or higher to make theparticles collide against the metal surface, thereby rapidly increasingand decreasing the temperature of the region close to the surface,thereby changing the state of the surface layer to increase the hardnessthereof, although this method does not relate to the contact member.

In addition, in Japanese Patent Application Laid-Open No. 4-331070(referred to as literature 3, hereinafter) which is laid open in 1992,there is proposed a method of elongating the lifetime of a tool byrefining and densifying the surface structure thereof by sprayingspherical abrasives having a grain size of 300 to 800 mesh onto thesurface along with a gas flow at a pressure of 3 to 10 kg/cm².

Furthermore, for the purpose of enhancing the fatigue strength of apower transmission mechanical component, such as a power transmissionaxis and a gear, Japanese Patent Application Laid-Open No. 61-124521(referred to as literature 4, hereinafter) which is laid open in 1986discloses a method of providing a product having a good surfaceroughness by austempering a steel to be processed to change the steelinto bainite, performing a first shot peening with a shot diameter of0.6 to 0.8 mm, a projection speed of 35 to 50 m/s and projectionduration of 5 to 40 ms to form a deep surface treatment layer, andfollowing the first shot peening in a moderate temperature range,performing a second shot peening under the same conditions, orpreferably with a smaller shot diameter of 0.3 to 0.5 mm, therebyfurther improving the compressive residual stress in the surface.

As for the prior arts described above, a plating technique for improvingcorrosion resistance has a problem that a large or large-scaleinstallation, such as a plating bath and a disposal installation, isrequired.

The technical field of the prior art disclosed in the literature 1 isnot relevant to the present invention, and although a particle collision(particle bombarding) treatment is used, the electrical contactivity isnot sufficiently improved because the treatment is performed only once.In addition, the literature 1 contains no description about corrosionresistance, and it can be considered that a sufficient corrosionresistance is not achieved by performing the treatment only once.

The technical fields and purposes of the prior arts disclosed in theliteratures 2 and 3 are not relevant to the present invention, and thereis a question as to whether the electrical contactivity is improved ornot. In addition, both the prior arts involve only one particlecollision treatment, and it can be considered that, if the prior artsare applied to a contact member, neither a sufficient electricalcontactivity nor a sufficient corrosion resistance are not achieved.

According to the prior art disclosed in the literature 4, the secondtreatment uses particles having a diameter smaller than particles usedin the first treatment. However, the technical field and purpose of theprior art is not relevant to the present invention, and therefore, itcannot be contemplated that this prior art is utilized to improve thecorrosion resistance and electrical contactivity of a contact member. Ifthis prior art is used to improve the corrosion resistance andelectrical contactivity of a contact member, a good electricalcontactivity cannot be achieved.

SUMMARY OF THE INVENTION

The present invention provides a surface modification method for aconductive metal material that improves the corrosion resistance andelectrical contactivity of the surface of the conductive metal material,a conductive metal material having improved corrosion resistance and ahigh electrical contactivity, a contact member made of the conductivemetal material, and a connector, which is an electrical connectingdevice, that has the contact member.

In a surface modification method for a conductive metal materialaccording to the present invention, a first modification treatment isperformed by bombarding the surface of the conductive metal materialwith spherical particles, each of which is made of a material that doesnot crack if the particle collides against the conductive metal materialand has a curvature approximately equal to or more than curvatures ofirregularities on the surface of the conductive metal material. Then, asecond modification treatment is performed by bombarding the surface ofthe conductive metal material subjected to the first modificationtreatment with spherical particles, each of which is made of a materialthat does not crack if the particle collides against the conductivemetal material and has a curvature less than curvatures ofirregularities on the surface of the conductive metal material subjectedto the first modification treatment.

A conductive metal material according to the present invention has anamorphous layer having a thickness of at least approximately 10 Å at thesurface thereof, the size of crystal grains of the conductive metalmaterial under the amorphous layer decreases as the distance from theamorphous layer decreases, and the surface of the amorphous layer isplanarized compared with a surface having irregularities that can becomea starting point of corrosion.

A contact member according to the present invention is made of theconductive metal material described above and can establish a goodelectrical connection and has a high corrosion resistance because thesurface is composed of an amorphous layer and more planarized than asurface having irregularities that can become a starting point ofcorrosion.

A connector according to the present invention his a contact sectionthat comprises the contact member described above, can establish a goodelectrical connection, and has a high corrosion resistance and a longlife.

In the surface modification method according to the present invention,the first modification treatment changes the entire surface of theconductive metal material into amorphous and reduces the curvatures ofirregularities thereon, and the resulting surface is planarized by thesecond modification treatment. Thus, a high corrosion resistance and ahigh electrical contactivity are achieved, and a large installation,such as a plating bath and a disposal installation, is not required.

BRIEF DESCRIPTION OF TIE DRAWINGS

FIG. 1 is a schematic diagram showing a configuration of an apparatusthat implements a method according to the present invention;

FIG. 2 shows a result of numerical analysis of a temperaturedistribution in the case where high-speed particles are made tosuccessively collide against a metal surface in a pulse-like manneraccording to a prior art;

FIG. 3 is a diagram showing a simulation of collision of sphericalparticles against the surface of a to-be-modified base materialaccording to the method according to the present invention;

FIG. 4 shows a partial cross section of the to-be-modified base materialwhose surface is modified and a change of crystal grain diameter alongthe depth of the to-be-modified base material;

FIG. 5 is a graph showing results of a corrosion resistance test, whichproves that the to-be-modified base material whose surface is modifiedaccording to the present invention has an improved corrosion resistance;

FIG. 6A is a cross-sectional view showing irregularities on the surfaceof the to-be-modified base material yet to be modified;

FIG. 6B is a cross-sectional view of the to-be-modified base materialthat is subjected to a modification treatment under an experimentalcondition 1;

FIG. 6C is a cross-sectional view of the to-be-modified base materialthat is subjected to a modification treatment under an experimentalcondition 2;

FIG. 6D is a cross-sectional view of the to-be-modified base materialthat is subjected to a modification treatment under an experimentalcondition 4; and

FIG. 7 is a perspective view of a connector according to the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS [Configuration ofApparatus]

First, a configuration of a surface modification apparatus for a basematerial that implements a method according to the present inventionwill be schematically described with reference to FIG. 1.

A stage 12 is disposed in a processing chamber 11, and a to-be-modifiedbase material (or a sample) 13 of a conductive metal material, which isused as a contact member, is placed on the stage 12. In this case, forexample, a reference position or a plurality of reference points areestablished, so that the to-be-modified base material 13 can bepositioned adequately on the stage 12.

Spherical particles 14 are ejected from a nozzle 15 onto theto-be-modified base material 13 to collide against the to-be-modifiedbase material 13. To this end, for example, a pressure tank 21, whichcontains the particles 14 charged thereinto by opening a lid 21 athereof, is disposed on the processing chamber 11. The lid 21 a isclosed tightly enough to withstand the pressure required to eject theparticles 14 from the tank 21 with a sufficient pressure. Each ofparticles 14 is made of a material that does not crack when it collidesagainst the to-be-modified base material 13 and has a spherical shape sothat it does not damage the to-be-modified base material 13. Thepressure tank 21 is in communication with a compressor 25 via a pipe 23,and the compressor 25 supplies a pressurized gas, such as compressedair, into the pressure tank 21 via a pressure regulator 24. In addition,the pressure tank 21 and the processing chamber 11 are in communicationwith each other via a pipe 26, and one end of the pipe 26 is introducedinto the pressure tank 21. A part of the pipe 26 distant from the end ofthe pipe 26 introduced into the pressure tank 21 is introduced into theprocessing chamber 11, and the other end of the pipe 26 constitutes anozzle 15.

The pipe 26 is made of a relatively flexible material so that a particleejection port of the nozzle 15 can be positioned with respect to anypoint on the stage 12.

A nozzle driving unit 31 is disposed outside the processing chamber 11,and a movable member 32 protrudes from the nozzle driving unit 31. Themovable member 32 is moved by a driving mechanism in the nozzle drivingunit 31 to any position on a two-dimensional coordinate system (or aplane). One end of a linkage bar 33 is fixed to the movable member 32. Apart of the linkage bar 33 distant from the end thereof fixed to themovable member 32 is introduced into the processing chamber 11 via anopening 11 a formed in the processing chamber 11, and the nozzle 15 isattached to the other end of the linkage bar 33.

An electrical control signal supplied from a control unit 34 is input tothe nozzle driving unit 31 via a cable 35, and the nozzle driving unit31 is controlled based on the control signal. The nozzle driving unit 31actuates the movable member 32 according to the control signal.Actuation of the movable member 32 causes the nozzle 15 to be moved to aposition determined by the control signal on the stage 12, or in otherwords, on the to-be-modified base material 13. In order to achieve suchmovement control, the reference position or point on the stage 12 andthe position of the movable member 32 that moves in the two-dimensionalcoordinate system are previously associated with each other. The nozzledriving unit 31 has a mechanism similar to that of the so-called XYstage driving unit 31.

The pressurized gas supplied from the compressor 25 is introduced intothe pressure tank 21 at a pressure regulated by the pressure regulator.Thus, the particles 14 in the pressure tank 21 are ejected from thenozzle 15 through the pipe 26 at a high speed. Each of particles 14ejected from the nozzle 15 collides against the to-be-modified basematerial 13, and each of particles 14 after collision is collected by adust collector 27 linked to the processing chamber 11 for reuse. Theclearance between the edge of the opening 11 a of the processing chamber11 and the linkage bar 33 is sealed with an elastic piece 11 b, such asa rubber piece, to prevent the particle 14 from escaping to the outsidewhile allowing free movement of the linkage bar 33.

Embodiment 1

According to the present invention, following a first modificationtreatment, a second modification treatment is performed. Before themodification treatments, fine irregularities, grain boundaries orlattice defects are found on the surface of a to-be-modified basematerial 13, and they are referred to collectively as irregularities inthis specification. For the first modification treatment, a sphericalparticle 14 is used that has a curvature approximately equal to or morethan the curvatures of the surface irregularities of the to-be-modifiedbase material 13. More specifically, a spherical particle 14 is usedthat has a diameter approximately equal to or less than the distanceacross the smallest recess of the irregularities on the surface of theto-be-modified base material 13.

In other words, a spherical particle 14 is used that has a curvatureapproximately equal to or more than the largest one of the curvatures ofthe irregularities on the surface of the to-be-modified base material13. Besides, as the spherical particle 14, for example, an aluminaparticle is used, which has a relatively high fracture toughness andtherefore can efficiently transmit the kinetic energy to theto-be-modified base material 13 without cracking when it collidesagainst the to-be-modified base material 13 at a high speed.

At room temperature, multiple spherical particles 14 are made tosubstantially successively collide against the surface of theto-be-modified base material 13 at high speeds. For example, it issupposed that the inner diameter of a nozzle 15 is 1.2 mm or more, theejection pressure is 3 kg/m² or higher, and the ejection duration is 10seconds or longer. Each of spherical particles 14, which collidesagainst the surface of the to-be-modified base material 13, has a highkinetic energy, so that the small point of impact is molten for a shorttime and rapidly cooled. The process is repeated for all the particles14.

This phenomenon will be more clearly understood from the followingdescription. For example, high-speed particles (spherical particles 14)are made to successively collide against the sample surface at a speedof about 200 m/s at intervals of 3 μs as pulses having a pulse width onthe order of nanoseconds. Then, the result of numerical analysis of theresulting temperature distribution is as shown in FIG. 2 (see NoboruEGAMI, “Fatigue Strength Properties of Surface Modification Steel byFine Particles Bombarding”, Surface Finishing Technology Vol. 52, No 2,1995, published by The Surface Finishing Society of Japan). That is,even if the spherical particles 14 are successively ejected from thenozzle 15, the spherical particles 14 collide against the surface of theto-be-modified base material 13 at random as shown in FIG. 3, forexample. At each small point of impact on the to-be-modified base metal13, a rapid heating and a rapid cooling process at a rate of 1000 K/μsor higher occur in a region extremely close to the surface, or to bespecific, to a depth of 1 μm or less. The rapid heating and rapidcooling process are repeated at intervals of about 3 μs, for example. InFIG. 2, the parameter Z denotes the depth from the surface.

During the first modification treatment, the rapid melting and the rapidcooling described above repeatedly occur on the surface of theto-be-modified base material 13. In the rapid cooling process, themolten metal solidifies before the atoms are arranged in position andtherefore is kept in a so-called amorphous state. That is, the minuteirregularities on the surface of the to-be-modified base material 13found before the first modification treatment are removed.

Then, according to the present invention, the second modificationtreatment is further performed. In the second modification treatment,each of spherical particles 14 having a diameter larger than that of thespherical particle 14 used for the first modification treatment, or moreprecisely, having a curvature smaller than the curvatures of theirregularities on the surface of the to-be-modified base material 13having been subjected to the first modification treatment is made tocollide at a high speed against the surface of the to-be-modified basematerial 13 having been subjected to the first modification treatment.In this case, except for the diameter, the conditions concerning thespherical particles 14 are the same as those concerning the sphericalparticles 14 used for the first modification treatment, and as theparticle, an alumina particle is used, for example.

The second modification treatment can be performed simply by replacing apressure tank 21 shown in FIG. 1 with a new one containing fineparticles used for the second modification treatment. To this end, it isprovided that pipes 23 and 26 can be removed from the pressure tank 21.Alternatively, the spherical particles 14 in the pressure tank 21 may bereplaced with the spherical particles used for the second modificationtreatment.

If the spherical particles 14 having curvatures smaller than those ofthe spherical particles used in the first modification treatment collideagainst the surface of the to-be-modified base material 13, through thesame mechanism as in the first modification treatment, the surfaceregion of the to-be-modified base material 13 is changed into amorphous,the corrosion resistance thereof increases, the irregularities havinglarge curvatures on the surface of the to-be-modified base material 13are reduced, the surface is planarized, and the contact resistance ofthe surface is reduced.

It is known that, if a metal surface has irregularities of largecurvatures, metal atoms at the irregularities have high chemicalpotentials, and such high chemical potentials cause corrosion of thesurface of the base material starting at the irregularities. To preventthis, the first modification treatment is performed to changeirregularities of significantly large curvatures into amorphous, therebyforming a natural oxide film through oxidation by oxygen in the air,that is, a passivation layer, on the surface thereof. However, the firstmodification treatment cannot enhance the electrical contactivity. Onthe contrary, in some cases, irregularities having larger curvaturesthan before the first modification treatment increase, and theirregularities have greater depth than before the first modificationtreatment. In other words, the first modification treatment may make thesurface coarser, and in this case, the electrical contactivity isdegraded. In addition, some of the irregularities of such increasedcurvatures tend to become a starting point of corrosion, and therefore,the corrosion resistance is not sufficiently improved. Thus, the secondmodification treatment is performed to planarize the surface having suchirregularities. Since the planarization mechanism is the same as that ofthe first modification treatment, the planarization changes the surfaceinto amorphous. Then, a passivation layer is formed on the resultingamorphous surface through oxidation by oxygen in the air, and thecurvatures of the irregularities on the planarized surface aresufficiently smaller than those of the irregularities that can be astarting point of corrosion. Therefore, both the corrosion resistanceand the electrical contactivity are improved.

FIG. 4 shows a cross section of the to-be-modified base material 13resulting from the two modification treatments and a change of crystalgrain diameter over regions of the to-be-modified base material 13. InFIG. 4, the vertical axis indicates the depth from a surface 13 a of theto-be-modified base material 13, and the horizontal axis indicates thecrystal grain diameter. A surface passivation layer 13 b 1 is formed bynatural oxidation to a depth d1 in the to-be-modified base material 13.Underlying the surface passivation layer 13 b 1, an amorphous layer 13 b2 is formed to a depth d2. Underlying the amorphous layer 13 b 2, arefined crystal layer 13 b 3 is formed in which the crystal grain sizeincreases with the depth from the surface. In the region at depths equalto or greater than a depth (13, there remains a base material layer 13 b4 in which the state of the crystal grains of the to-be-modified basematerial 13 before the modification treatments is kept unchanged.

The spherical particle 14 used in the first modification treatment has asize enough to change the projection or recess of the largest curvatureon the surface of the to-be-modified base material 13 before surfacemodification into amorphous. For example, it is enough that thespherical particle 14 has a diameter equal to or less than the distanceacross the projection or recess of the largest curvature of theirregularities on the surface before surface modification that can be astarting point of corrosion. When such a small particle is used, theejection pressure is increased to achieve the repeated rapid melting andrapid cooling of the surface described above. That is, the size of thespherical particle 14 and the ejection pressure are determined so thatthe rapid melting and cooling can be achieved.

As for the second modification treatment, the quality of planarizationincreases with the size of the spherical particle 14, as describedabove. However, actually, the second modification treatment isrestricted in terms of the availability of the particle, the innerdiameter of the nozzle 15 or the like.

Experiment

Using phosphor bronze as the to-be-modified base material (simplyreferred to as sample, hereinafter), comparison experiments wereperformed under four different conditions described below. An aluminaparticle with relatively high fracture toughness was used as theparticle 14, and the shape of the particle 14 was substantiallyspherical so that it did not etch the surface of the sample. Thediameter of the nozzle 15 was 1.5 mm, the particle ejection pressure was8 kg/cm², the ejection duration was 1 minute, and compressed air at roomtemperature was used for ejection of the particle. Here, except for thediameter of the particle 14, the first modification treatment and thesecond modification treatment were performed under the same condition.

Condition 1: the surface is treated only once using a particle having adiameter of 20 μm (comparison experiment).

Condition 2: the surface is treated only once using a particle having adiameter of 50 μm (comparison experiment).

Condition 3: after the surface is treated using a particle having adiameter of 50 μm, the surface is treated again using a particle havinga diameter of 20 μm (comparison experiment).

Condition 4: after the surface is treated using a particle having adiameter of 20 μm, the surface is treated again using a particle havinga diameter of 50 μm (embodiment 1 of the present invention).

After the surface modification treatment was performed under each ofthese conditions, a salt spray test was performed for 48 hours on eachof the surface-modified samples to evaluate the corrosion resistancethereof. In addition, the salt spray test was performed also on aphosphor bronze base material (sample) that was not subjected to anysurface modification treatment. In this case, the surface was completelyrusted, and the contact resistance was significantly reduced. Based onthis fact, it could be seen that the corrosion resistance improvement bythe surface modification could be assessed by the salt spray test.

After the 48 hours of salt spray test, using a contact-resistance meterwith a gold probe, the contact resistance of each sample was measuredwhile changing the load from 1 g to 100 g. FIG. 5 shows the result. InFIG. 5, the horizontal axis indicates the load, and the vertical axisindicates the contact resistance. The measuring range of the contactresistance meter is from 0 mΩ to 20 mΩ. A curve 41 (solid line), a curve42 (alternate long and short dash line), a curve 43 (dashed line) and acurve 44 (dotted line) indicate the experimental results for the samplestreated under the conditions 1, 2, 3 and 4, respectively. In addition, acurve 45 (alternate long and two short dashes line) indicates the resultof measurement of the contact resistance of the phosphor bronze basematerial that is not subjected to neither any surface modificationtreatment nor any salt spray, which is performed by similarly changingthe load. It can be said that the curve 45 indicates a state of thesample that is suitable for commercialization.

As indicated by the solid line 41, the sample treated under thecondition 1 exhibited a contact resistance equal to or higher than 20mΩ, which was the measurement limit, over the whole range of load andtherefore was not suitable for commercialization. The reason for thiswill be described with reference to FIG. 6A, which is a cross-sectionalview showing irregularities on the surface of a sample 60 yet to besurface-modified. In the modification treatment under the condition 1,the sample surface was bombarded with each of particles 14 _(s) having acurvature larger than that of a recess 61 having the largest curvaturebefore the modification treatment, that is, the smallest recess. Asshown in FIG. 6B, for example, the modification treatment formed anamorphous layer 62 on the sample surface and many irregularities on thesample surface, although the irregularities had curvatures smaller thanthat of the recess 61. The irregularities are considered as a cause ofthe degradation of the contact resistance. In addition, irregularitiesof such curvatures can be a starling point of corrosion.

On the sample treated under the condition 2, there ware observed manycorrosions starting from small recesses that the particle having adiameter of 50 μm couldn't get into. As a result, as indicated by thealternate long and short dash line 42, the contact resistance wassignificantly worse than before salt spray (see the alternate long andshort two dashes line 45). In the modification treatment under thecondition 2, the sample surface was bombarded with each of particles 14_(L) having a curvature smaller than those of irregularities on thesurface before the modification treatment. Therefore, as shown in FIG.6C, the interior of the recess 61 was not changed into amorphous by theparticles 14 _(L). Therefore, the recess 61 served as a starting pointof corrosion, so that the corrosion resistance was degraded. In otherwords, it can be considered that the interior of the recess 61 remainedunmodified, and corrosion started from the recess 61 to degrade thecontact resistance.

As for the sample treated under the condition 3, while the surface wasplanarized by the first modification treatment, small recesses remainedon the surface as in the case of the condition 2. In addition,additional irregularities ware formed on the surface by the secondtreatment. Thus, as for the sample treated under the condition 3, it canbe considered that not only the contactivity was degraded, but alsocorrosion occurred and the contact resistance was degraded as indicatedby the dashed line 43.

Compared with the conditions 1 to 3 described above, the sample treatedunder the condition 4, that is, treated according to the embodiment 1 ofthe present invention exhibited an extremely preferred contactresistance, as indicated by the dotted line 44. This can be explained asfollows. Because the entire surface was first modified along theirregularities using the particles having a diameter of 20 μm, that is,the entire surface was modified as shown in FIG. 6B, and then, theparticles 14 _(L) having a diameter of 50 μm, which had a curvaturesmaller than those of the irregularities on the surface, collidedagainst the surface, irregularities having small curvatures wareplanarized as shown in FIG. 6D, for example, the amorphous layer 62 wasformed over the entire surface, and the surface had a high corrosionresistance and a high electrical contactivity. The curve 44 indicatingthe result of treatment of the sample under the condition 4 approximatedto the curve 45. In other words, the contact resistance of the sampletreated under the condition 4 was not significantly degraded, comparedwith the contact resistance of the unmodified phosphor bronze samplebefore salt spray which was a good sample for comparison.

The experimental results described above prove that the treatment underthe condition 4 (according to the embodiment of the present invention)is extremely effective in improvement of corrosion resistance, comparedwith the treatments under the conditions 1 to 3. While particles havinga diameter of 20 μm ware used as smaller particles 14 and particleshaving a diameter of 50 μm ware used as larger particles 14, of course,any particle diameter can be chosen depending on the size ofirregularities on the surface of the to-be-modified base material foruse.

Referring to the cross-sectional view of FIG. 4, in the case of thesample treated under the condition 4, the thickness of the passivationlayer 13 b 1 was 10 Å or less, the thickness of the amorphous layer 13 b2 was 10 Å or more, and the total thickness of the amorphous layer 13 b2 and the refined crystal layer 13 b 3, that is, the thickness of thesurface-modified layer was about 10 μm.

It is desirable that the thickness of the amorphous layer 13 b 2 isequal to or more than 10 Å. This is based on the descriptions that thethickness of the passivation layer is approximately 10 Å (“PracticalKnowledge about Corrosion and Corrosion Prevention”, InternationalStandard Book Number (ISBN): 4-274-08721-2, p. 10) and that thethickness of the passivation layer of a corrosion-resistant metalreferred to as passivity metal is approximately 10 Å (“Handbook ofElectrochemistry”, International Standard Book Number (ISBN):4-621-04759-0, p. 427). The amorphous layer 13 b 2 has a higher volumeresistivity than the refined crystal layer 13 b 3 and the base materiallayer 13 b 4 and, thus, preferably has a small thickness. Up to athickness of passivation layer of about 100 Å, a sufficiently practicalcontact resistance can be achieved due to the tunnel effect even in thecase of an insulating film. Provided that the effective area of thecontact is 0.1 by 0.1 mm², and degradation of the volume resistivity ofthe amorphous layer 13 b 2 can be allowed up to two orders of magnitude,a sufficiently practical contact resistance can be achieved with theamorphous layer 13 b 2 having a thickness up to 1 μm.

If in the underlying layer the natural oxide film has only a fewfailures, the reaction does not proceed, and the natural oxide filmconstitutes a passivation layer. The thickness of the passivation layeris not greater than approximately 10 Å.

In the case of phosphor bronze, the irregularities on the surface yet tobe modified, that is, the crystal grains have sizes from 10 to 20 μm.The diameter of the particle 14 used in the first modification treatmentis preferably approximately equal to or less than 20 μm so that thesmallest recess, that is, the recess having the largest curvature can bechanged into amorphous. The diameter of the particle 14 used in thesecond modification treatment is preferably more than 20 μm. Thematerial for the contact member is not limited to phosphor bronze andmay be brass, Corson copper or the like.

Embodiment 2

FIG. 7 shows a connector according to an embodiment 2 of the presentinvention. A connector (a plug in this example) 51 is mounted on awiring board 50. A rectangular-parallelepiped housing 52 of theconnector 51 is attached to the wiring board 50 along one edge thereof.From a surface of the housing 52 that extends along that edge and isperpendicular to the wiring board 50, two rows of pin-shaped contacts53, whose surfaces are modified according to the present invention,protrude to the outside of the wiring board 50 in parallel with theplane of the wiring board 50. Although not shown, each pin-shapedcontact 53 is connected to a wiring on the wiring board 50.

A socket connector 54, which is a counterpart of the plug connector 51,has a contact section composed of contact members, whose surface ismodified according to the present invention, housed in a contact housingopening (not shown) of a housing 55 thereof. The pin-shaped contacts 53can be connected to or disconnected from the contact section composed ofcontact members by inserting or removing the pin-shaped contacts 53 intoor from the contact housing opening. A lead wire 56 having one endconnected to each contact member of the socket connector 54 is led fromthe housing 55 to the outside.

Since the contact members have surfaces modified according to thepresent invention, the connectors establish a good electrical connectionand have a high corrosion resistance and a long life. The pin-shapedcontacts of the plug connector may be perpendicular to the wiring board50. Furthermore, the present invention can be applied to various casesin which a different number of rows of contacts, a different number ofcontacts or the like is used.

1. A surface modification method for a conductive metal material,comprising: a first modification step of bombarding the conductive metalmaterial with spherical particles, each of which is made of a materialthat does not crack if the particle collides against the conductivemetal material and has a curvature approximately equal to or more thancurvatures of irregularities on the surface of the conductive metalmaterial; and a second modification step of bombarding the surface ofthe conductive metal material treated in the first modification stepwith spherical particles, each of which is made of a material that doesnot crack if the particle collides against the conductive metal materialand has a curvature less than curvatures of irregularities on thesurface of the conductive metal material treated in the firstmodification step.
 2. The surface modification method for a conductivemetal material according to claim 1, wherein the conductive metalmaterial is phosphor bronze, brass or Corson copper. 3-5. (canceled)